Orthogonal Frequency Division Multiple Access (OFDMA) technology is getting very popular in modern communication systems since the OFDMA technology can efficiently support multiple mobile stations with limited bandwidth and easily provide Quality of Service (QoS). The OFDMA technology is a multiple access version of orthogonal frequency-division multiplexing (OFDM). OFDM is a modulation technique for data transmission based on frequency-division multiplexing (FDM), which uses different frequency channels to transmit multiple streams of data. In OFDM systems, a wide channel is divided into multiple narrow-band subcarriers, which allow orthogonal modulated streams of data to be transmitted in parallel on the subcarriers.
At an OFDM transmitter, outgoing information is assembled into blocks of N complex number, one for each subcarrier. An N-point inverse fast Fourier transform (IFFT) is performed on each block, and the resultant time domain signal is transmitted. Usually, several blocks are grouped to form a frame, and one extra block with known pattern, which is referred to as “preamble”, is inserted into every frame for acquisition and channel estimation purposes.
At an OFDM receiver, a digital automatic gain control (AGC) circuit measures the strength of the base-band signal at an analog-to-digital (ADC) output, and adjusts the gain to an appropriate level for a range of input signal levels based on the average signal strength of the preamble. There are a number of factors that affect the design of a digital AGC circuit, where performance, speed and complexity are the three main factors.
Multiple-input multiple-output (MIMO) in wireless links is rapidly becoming the new frontier of wireless communications as well. MIMO is an antenna technology for wireless communications in which multiple antennas are used at both the transmitter and the receiver. In properly designed MIMO systems, where there are Nt antennas at the transmitter and Nr antennas at the receiver, Nt times Nr independent air-links are available so that significant diversity gain, spatial multiplexing gain, and array gain are possible. Traditional single antenna system is usually referred to as SISO (single-input single-output).
MIMO OFDMA system is a combination of both state-of-the-art technologies. To realize the significant performance gain at the receiver's side, a MIMO AGC should have the capability to track the strength of the signals from all antennas. Complexity is a major issue for a MIMO AGC.
Thus, there is a need for a digital multi-chain AGC circuit for an OFDM receiver having good performance and convergence speed with reduced complexity.